Process of refining hardwood pulp



Aug. 24, 1954 B. G. Hoos 2,687,352

PROCESS OF REFINING HARDWOOD PULP Filed Feb. 21, 1946 s Sheets-Sheet 1INVENTOR.

r //-vS 1 Qw ION Stock-Densz'i'y.

2.0 5.0 L CAUSTIC SODA EOLUTION T U L Q s A D O 5 m m U A P L m W m FERCENT I NITM 0 000000 MM8W65 32| I Aug. 24, 1954 B. G. Hoos 2,587,352

PROCESS OF REFINING HARDWOOD PULP Filed Feb. 21, 1946 3 Sheets-Sheet 2PER CENT INITIAL CAUSTIC SODA SOLUTION PER CENT INITIAL STOCK DENSITYIll] u. I00 I 0 90 u fi an m g? 70 065%S1ockflensli'y. mo 2 a 60 +10% 2150 0/57; In 5 x252 3 CL 40 5 o 5!] Lu g 20 l0 4 D 0 0 .l 0.2 0.3 0.4INVENTOR.

F'ER' CENT ETHEH-SOLUBLE CONTENT OF F'ULF.

Aug. 24, 1954 B. G. HOOS 7,

PROCESS OF REFINING HARDWOOD PULP Filed Feb. 21, 1946 3 Sheets-Sheet 3CAUSTIC ABSORBENCY INSTABILHY FACTOR OF PULP aw m f n 0 .05 .10 J5 .20.25 .50 .55 PER CE NT ETHER-E OLUBLE CONTENTQF PUL INVEN TOR.

BY aw M 1:

@W WZQM Patented Aug. 24, 1954 UNITED srArs erENT OFFICE PROCESS OFREFINING HARDWOOD PULP Benjamin G. Hoos, Berlin, N. H., assignor, bymesne assignments, to Brown Company, Portland, Maine, a, corporation ofMaine Application February 21, 1946, Serial No. 649,297

6 Claims. 1

This invention relates to the productionfrom hardwoods, such as whitebirch, yellow birch, maple, beech and the like, of a cellulose pulpproduct useful in the production of cellulose derivatives, particularlycellulose ether, and is concerned specifically with the production forsuch use of refined alpha pulp from sulfite or sulfate stock constitutedof the above type of hardwoods.

Quality of cellulose derivatives is directly alfected by thecharacteristics of the source of cellulose from which the derivativesare produced. Softwood pulps, such as refined spruce sulfite pulp, maybe readily produced which have the necessary characteristics for highgrade cellulose derivative production and for the production ofcellulose ethers, but, prior to this invention there has been no knownhardwood pulp which could be satisfactorily substituted in these usesfor the softwood pulps, despite the lower cost advantage which wouldresult from such substitution. Hardwood pulps have been whollyunacceptable in continuous cellulose ether processes.

Softwood pulps of the above type are acceptable for these uses becauseof their low ethersoluble resin content. However no known process ofrefining or otherwise treating pulp has been capable of producing thenecessary low resin content in the case of hardwood stock asdistinguished from softwood stock.

Reduction of resin content in hardwood pulprefinement is dealt with inRichter Patent No. 2,228,127. This patent proposed as a remedy forexcessive lignin and/or resin content subjecting preliberated hardwoodpulp to a multi-stage sequence which included an initial hypochloritetreatment to prevent excessive development of resistance in the resinsof the pulp. -In such manner the patentee was successful by. amulti-stage prebleach-chlorinationhot caustic soda-soapdigestionalkaline bleach sequence in realizing ether-solubleconstituentsor resin content of only about 01 to 0.2%, as shown inExample 1 of that patent.

My experience has shown that, while pulp with these ether-solublecontents has valuable uses, its ether-soluble content is still too highfor manufacture of high quality cellulose derivatives. In addition, suchpulps are not adapted for use in continuous cellulose ether processesbecause they have too slow absorption rate for the caustic solutionstherein used. No modification in the Richter multi-stage process hasproduced a hardwood pulp possessing absorbency rates for causticssufficiently high, after a usual storage period, to permit itsacceptance as a reliable source material in such continuous processes.

In fact, prior to this invention, such. qualities as unacceptably highresin content and poor absorption of caustic solutions have lefthardwood pulp products, as a class, unadapted as materials for highquality cellulose derivative production or for any cellulose derivativeproduction which involves caustic treatment of the pulp as in theproduction of cellulose ethers.

The primary object of this invention is to produce a hardwood pulp,particularly of mixed hardwoods, which has, along with other acceptablequalities, including an alpha cellulose content exceeding 92.5%, acaustic solution rate of absorption which renders the pulp useful incommercial cellulose derivative processes involving caustic solutiontreatment and which absorption rate is relatively independent of time sothat storage intervals do not affect the desired absorbency quality to apoint beyond a practically useful limit, thus lending the pulp to use asan improved source for cellulose other production. Hardwood pulps ofthis invention also have exceedingly low ether-soluble contents neverbefore attained in the pul industry, and which lend the pulp towidespread use in general cellulose derivative production, as a lowercost substitute for softwood pulp, even when a high rate of causticabsorption is not an essential quality. It has been established thatcaustic absorption of the pulp is directly related to ether-solublecontent so that by precise control of ether-soluble content, desiredcaustic'absorbency rates are simultaneously secured. More broadly, then,an object of this invention is the production of a hardwood pulp whichis substantially free of ether-soluble resin content.

The invention has for a further object the provision of a process foryielding hardwood pulp of the above characteristics.

For the purposes of this application, certain terms are herein used withthe following meaningsz Caustic absorbency connotes the time interval inseconds between the deposit and complete absorption of a drop of 35%NaOH solution at 25 C. from a 5 diameter glass rod, rounded on the end,on and by a simulated dryer sheet of about 300 basis (weight in poundsof 480 sheets, 24 inches by 36 inches) prepared by pressing 5 inchdiameter wet sheets under a one ton weight and then drying on a drumheated by steam at 20 pounds gauge steam pressure, the sheets beingturned every two minutes.

Initial caustic absorbency is caustic absorbency before storage of thesheets.

Ultimate caustic absorbency is caustic absorbency after the simulateddryer sheets have been submitted to an arbitrary accelerated aging byheating for 16 hours at 100 C.

Caustic absorbency instability factor is the ratio of ultimate toinitial caustic absorbency.

In accordance with this invention, hard-wood pulp products are formedhaving ether-soluble contents less than 0.10%, ultimate causticabsorbency values less than about 40 seconds, which represents apractical maximum absorbency value acceptable for commercial celluloseether production, initial caustic absorbency values preferably less thanseconds, and caustic absorbency instability factors not greater thanabout 2.8.

It has been found that these desirable characteristics may be attainedin hardwood pulp products by establishing in a three-stage refiningsequence involving prebleach-hot caustic soda soap digestion-alka1inebleach, novel relations of stock density and caustic solutionconcentration in the digestion step.

An example of preferred procedure is as follows:

Wood chips made from mixed hardwoods such produce 10% stock density anda 2.2% NaOH and 0.33% soap concentration, both based on the weight ofthe liquor. The pulp was then digested at 205 to 210 F. for 5 hours. Thepulp was then washed, thickened to 12% stock density and treated from 3to 4 hours at 110 F. with 0.7% chlorine equivalent as sodiumhypochlorite plus 0.25% NaOH, based on the weight of the pulp, at 10%stock density. The pulp was then washed, treated with a dilute S02solution to neutralize residual chlorine and then washed again. Thewashed pulp was then formed into simulated dryer sheets.

For purposes of comparison, a control was established wherein the onlyvariant was the use of l%.% NaOH solution which is about the customaryconcentration, instead of 2.2% NaOH solution.

The simulated dryer sheets had the following characteristics Theultimate caustic absorbency of the control sample, 595 seconds, way overthe maximum of =10 seconds, is so poor as to render this pulp whollyunacceptable for use in commercial cellulose ether production involvingcontinuous saturation of the pulp sheet with a caustic soda solution. Onthe contrary, the initial and ultimate caustic absorbency values of 15and 34 seconds, respectively, of the higher caustic usage sample arewithin the range suitable for use in such production. Furthermore, therelatively high ether-soluble content of the control sample renders thepulp unsuitable for use in the production of any high quality cellulosederivatives.

As the above example illustrates, ultimate caustic absorbency is broughtwithin the range of usefulness for cellulose ether production andether-soluble content within the desired low range by utilization ofexcess caustic during as white birch, yellow birch; beech and maple 5Cdigestion at the 10% stock density of the ex- Were cooked in asodium-base sulfite cooking acid amp e. containing 5% free S02 and 1%combined S02. Experiments indicate, however, that there is a Theliberated fiber was washed, screened and fi e a ip. in the production ofP 1 thickened. The unbleached fiber was then having the preferredcharacteristics of this intreated at 4% stock density for 30 minutes atvention, between initial stock density and initial F. with theequivalent of 1.4% chlorine, based caustic concentration in thedigestion sequence. on pulp, as sodium hypochlorite. This pre- Theefiect of variation of caustic per cent at bleached fiber was washed andthickened to 10% stock density upon ultimate caustic absorbabout 14%stock density. The thickened fiber ency is shown in the following table,the process was then mixed with suflicient 25% caustic soda 00 otherwiseremaining as stated in the example solution, 10% soap solution, and hotwater to given above:

Table II I 't'al Ultimt- Ca t' 53% Percent 3235 332? o tuc Caust ic Absol l lfb lfl cy Solution Stock Solution Soluble g ggg gg figgg gg iaggg 1. s3 10 as o. 27 25 595 23. s 1. e7 10 .33 0. ll 31 269 s. 1 1. 941o 33 0. 09 29 104 a. s

2.22 10 .33 0.09 15 a4 2.3 a. s3 10 a3 0. 04 7 11 1. 6 4. 44 1c .33 0.04 7 10 1. 4 5.55 10 .33 0.06 5 7 1.4 6.66 10 .33 0.08 5 9 1.8

. Examples appearing below the horizontal line have ultimate causticabsorbency values within the desired low range, as contrasted to thehigher unsatisfactory values of those appearing above the horizontalline.

The effect of variation in stock density at different percentages ofNaOH are shown in the following table, the process again remainingotherwise the same, and the horizontal lines again segregating, in thecase of each stock density,-

caustic per cent solution producing desirable and undesriable ultimatecaustic absorbencies.

Table III Initial Ultimate ggggfifi Percent Percent Percent PercentCaustic Caustic ency Stock NaO H Soap ther Absorb- Absorb- InstwSolut1on Solution Soluble ency m ency 1n bflity Seconds Seconds Factor6. 25 l. 67 33 0. 07 8. 9 25 2. 8 6. 25 1. 94 33 0. 03 6. 1 12. 75 2. l6. 25 2. 22 33 0. 02 6. 1 12. 75 2. 1 6. 25 3. 33 .33 0.02 6.1 11 1.8 6.25 4. 44 33 0. 03 5. 4 10. 25 1. 9 1G 1. 33 33 0. 18 10. 0 83 8. 3 16 1.67 83 0. 18 10. 4 51 4. 9 16 l. 94 33 0. 11 10. 5 42 4. 0

Certain relations shown in the above tabulations are plotted in thegraphs shown in the accompanying drawings, in which Fig. 1 shows curvesof initial caustic soda concentration in per cent plotted againstpercentage ether-soluble content of the pulp, for 6.25, 10, 16 and 25%initial stock densities, respectively;

Fig. 2 shows curves of initial caustic soda concentration in per centplotted against ultimate caustic absorbency values in seconds, for 6.25,10, 16 and 25% initial stock densities, respectively;

Fig. 3 shows a curve of initial stock density plotted against per centcaustic solution defining the lower limit of ratios therebetweennecessary for the production of the pulps of this invention;

Fig. 4 is a graphical representation of percentage ether-soluble contentof the pulp plotted against ultimate caustic absorbency values inseconds, for 6.25, 10, 16 and 25% initial stock densities, respectively,and indicating in broken lines the approximate maximum values forether-soluble content and/or ultimate caustic absorbency in pulps ofthis invention; and

Fig. 5 is a curve similar to that shown in Fig. 4 except that theether-soluble content is plotted in Fig. 5 against caustic absorbencyinstability factor instead of against ultimate caustic absorbency as inFig. 4 and again showing in broken lines the approximate maximum valuesfor the production of pulps in accordance with this invention.

As indicated by the curve of Fig. 1, ether-soluble content dropsexceedingly rapidly with increase in initial per cent caustic usage andafter reaching a certain critical initial caustic usage for each stockdensity producing ether-soluble contents less than-0.10%,ether-solublecontent values are 0.10% ether-soluble content abscissa of Fig. 1. Thecurve AB of Fig. 3 thus represents the lower limit of initial stockdensity to per cent caustic solution ratio permissible for theproduction of preferred pulps of this invention, a plotted ratio, whenfalling in the area above the curve AB of Fig. 3, producing such a pulpand when falling in the area on or below the curve AB, not producingsuch a pulp.

In Fig. 4, values shown in Figs. 1 and 2 are plotted to show per centether-soluble content against ultimate caustic absorbency in seconds.Pulps of this invention have ratios which, when plotted on the graph ofFig. 4, fall to the left of the vertical broken line or under thehorizontal broken line or both. A similar segregation of satisfactoryand unsatisfactory pulps is represented in Fig. 5 wherein theether-soluble content is plotted against caustic absorbency instabilityfactor in lieu of ultimate caustic absorbency in seconds. Pulps of thisinvention have values falling to the left of the vertical dot and dashline or under the horizontal dot and dash line or both. I As shown bythe curve AB of Fig. 3, for initial stock density values greater thanabout 14% it is essential, to produce preferred pulps of this invention,to utilize caustic solutions in excess of 2% and progressivelyincreasing. Below about 14% initial stock density, as indicated by thecurve AB, the caustic solution per cent may go below 2% and stillproduce satisfactory low ethersoluble content pulps.. In fact, theminimum caustic solution percentage for values of initial stock densityless than about 14% are related to the amount of excess caustic presentover that necessary to refine the pulp. The efiect of caustic excess forvalues of initial stock density of 6.25

and 10% initial stock densities are illustrated in the followingtabulation:

Table IV Initial Caustic Residue Percent Percent Caustic Residue PercentEther- Stock Percent of Percent of of Caustic Soluble Solution SolutionInput Content 10 1. 67 0. 65 39 0.14 10 l. 33 O. 30 22. 6 0. 27 6. 25 2.22 1. 54 69. 4 0. (32 6.25 1. 94 l. 25 64. 5 0. 03 6. 25 1. 67 1.01 60.5 0. 07

As shown by the above table, at initial stock densities of 10 and 6.25%,respectively, ethersoluble contents less than 0.10% are produced only inthose cases (appearing above the horizontal lines in Table IV) where theper cent caustic residue based on initial caustic input is exceedinglyhigh as compared with the residues encountered in normal refiningprocesses, which are always maintained as low as possible and rarely, ifever, exceed 10%. For practical purposes of this invention the residuein the case of initial stock densities less than about 14% must exceed25% of the initial caustic input and running in some cases at the lowerdensities, as shown by the above table, upwards of 60% whereether-soluble contents materially below 0.05% are desired.

In practice, the waste liquors derived from digestion of pulps of thisinvention, when relatively concentrated both in caustic and the moreexpensive soap may be recycled by being fortified with additional soapand caustic without any detrimental result on the product. Despitecontinued recycling use, no loss in the efficiency of the process hasbeen found.

With regard to soap usage, the concentration should be in excess ofabout 0.11% and preierably the concentration is 0.20% or greater. Soapconcentrations of 0.33% have been found to be particularly suitable. Theeffect of variation in soap concentration at 10% stock density is shownby the following table:

Table V i nitial "iglltimrte Gaugtic Percent Percent Percent austm ansAbsor Absorb- Absorb- Insta- Soap NaOH Stock ency in ency in bilityseconds seconds seconds 11 2. 22 I 10 36 450 12. 0. 33 2. 22 i l0 l4. 834 2. 3 0. 56 2. 22 13. 5 22 l. 6 0. 33 l. 33 10 25 595 23. S 0. 56 l.33 10 27. 5 393 14. 3

The resin or ether-soluble content of the pulp given herein isdetermined by the standard method for ether extraction known as T204-mof the Technical Association of the Pulp and Paper Industry.

I claim:

1. The process of refining preliberated hardwood pulp to ether-solublecontents less than 0.10% in a multistage chemical refining process,including a hot caustic-soap stage, comprising subjecting the hardwoodpulp, in the hot causticsoap stage, to hot caustic :soda refining liquorhaving @a soap concentration between about 0.11% and 0.56% based on thesolution, and regulating the initial stock consistency in said stagebetween about 6.25% and about 25% and the concentration of caustic sodain the refining liquor in said stage between about 1.54% and 6.66% basedon the solution, so that in a graph plotting the per cent initial stockconsistency against the per cent initial caustic soda based on thesolution, the per cent of caustic soda based on the solution lies abovea straight line extending through the plots for about 1.54% caustic sodaconcentration at 6.25% stock consistency and 2.7% caustic sodaconcentration at 25% stock consistency.

2. A process of refining preliberated hardwood pulp to ether-solublecontents less than 0.10% as claimed in claim 1, wherein the initialstock consistency is between about 14% and about 25% based on thesolution.

3. A process of refining preliberated hardwood pulp to ether-solublecontents less than 0.10% as claimed in claim 1, wherein the said soapconcentration is about 0.33%.

4. A process of refining preliberated hardwood pulp to ether-solublecontents less than 0.10% as claimed in claim 1, wherein theconcentration of caustic soda in the refining liquor in said stage isbetween about 1.54% and 4.44% based on the solution.

5. The process of refining preliberated hardwood pulp to ether-solublecontents less than 0.10% in a multistage chemical refining process,including a hot caustic-soda stage, comprising subjecting the hardwoodpulp, in the hot causticsoap stage, to hot caustic soda refining liquorhaving a soap concentration between about 0.11% and 0.56% based on thesolution, and regulating the initial stock consistency in said stagebetween about 6.25% and about 25% and the concentration of caustic sodain the refining liquor in said stage between about 1.54% and 4.44% basedon the solution, so that in a graph plotting the per cent initial stockconsistency against the per cent initial caustic soda based on thesolution, the per cent of caustic soda based on the solution lies abovea straight line extending through the plots for about 1.54% caustic sodaconcentration at 6.25% stock consistency and 2.7% caustic sodaconcentration at 25% stock consistency, the hardwood pulp beingsubjectcd to said refining liquor at a temperature of about 205-210 F.for a period of about five hours.

6. A process or" refining preliberated hardwood pulp to ether-solublecontents less than 0.10% in a multistage chemical refining process,including a hot caustic-soap stage, comprising subjecting the hardwoodpulp, in the hot caustic-soap stage, to hot caustic soda refining liquorhaving a soap concentration between about 0.11% and 0.56% and an initialcaustic soda concentration between 2.22% and 6.66%, both based on thesolution, at a stock consistency of about 10% and temperature of about205-210 F. for a period of about five hours.

References Cited in the file of this patent UNITED STATES PATENTSliumber Name Date 1,806,309 Richter May 19, 1931 1,833,976 Richter Dec.1, 1931 1,847,311 Schur Mar. 1, 1932 1,945,202 Richter Jan. 30, 1934{Other references on following page) 10 UNITED STATES PATENTS OTHERREFERENCES Number Name Date Industrial and Engineering Chemistry, vol.23,

1,947,106 Plumstead Feb. 13, 1934 N0. 2, pages 138 and. 139. 1,949,549Richter Mar. 6, 1934 Bulletin U. S. D. A. No. 80, Aug. 31, 1914, pages1,991,499 Drewsen Feb. 19, 1935 5 1s, 19 and 22. 2,030,383 Luth et a1Feb. 11, 1936 Refining of Pulp by Rys and Bonisch, Paper 2,041,958Richter Mar. 26, 1936 Trade Journal, May 11, 1939, pages 31 to 40.2,118,039 Dreyfus May 24, 1938 The Hot Alkaline Purification ofCellulose I by 2,159,675 Richter May 23, 1939 Meller Paper TradeJournal, Sept. 27, 1945, pages 2,228,127 Richter Jan. 7, 1941 133 to145.

0 Gray Aug. 15, 1950 Cellulose and Cellulose Derivatives by Ott,

published by Interscience Publishing, New York

1. THE PROCESS OF REFINING PRELIBERATED HARDWOOD PULP TO ETHER-SOLUBLECONTENTS LESS THAN 0.10% IN A MULTISTAGE CHEMICAL REFINING PROCESS,INCLUDING A HOT CAUSTIC-SOAP STAGE, COMPRISING SUBJECTING THE HARDWOODPULP, IN THE HOT CAUSTICSOAP STAGE, TO HOT CAUSTIC SODA REFINING LIQUORHAVING A SOAP CONCENTRATION BETWEEN ABOUT 0.11% AND 0.56% BASED ON THESOLUTION, AND REGULATING THE INITIAL STOCK CONSISTENCY IN SAID STAGEBETWEEN ABOUT 6.25% AND ABOUT 25% AND THE CONCENTRATION OF CAUSTIC SODAIN THE REFINING LIQUOR IN SAID STAGE BETWEEN ABOUT 1.54% AND 6.66% BASEDON THE SOLUTION, SO THAT IN A GRAPH PLOTTING THE PER CENT INITIAL STOCKCONSISTENCY AGAINST THE PER CENT INITIAL CAUSTIC SODA BASED ON THESOLUTION, THE PER CENT OF CAUSTIC SODA BASED ON THE SOLUTION LIES ABOVEA STRAIGHT LINE EXTENDING THROUGH THE PLOTS FOR ABOUT 1.54% CAUSTIC SODACONCENTRATION AT 6.25% STOCK CONSISTENCY AND 2.7% CAUSTIC SODACONCENTRATION AT 25% STOCK CONSISTENCY.